.\" SPDX-License-Identifier: Linux-man-pages-1-para
.\"
.\" This man page is Copyright (C) 2000 Andi Kleen <ak@muc.de>.
.\"
.\" $Id: ipv6.7,v 1.3 2000/12/20 18:10:31 ak Exp $
.\"
.\" The following socket options are undocumented
.\"	All of the following are from:
.\"	    commit 333fad5364d6b457c8d837f7d05802d2aaf8a961
.\"	    Author: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>
.\"	    Support several new sockopt / ancillary data in Advanced API (RFC3542).
.\"		IPV6_2292PKTINFO (2.6.14)
.\"		    Formerly IPV6_PKTINFO
.\"		IPV6_2292HOPOPTS (2.6.14)
.\"		    Formerly IPV6_HOPOPTS, which is documented
.\"		IPV6_2292DSTOPTS (2.6.14)
.\"		    Formerly IPV6_DSTOPTS, which is documented
.\"		IPV6_2292RTHDR (2.6.14)
.\"		    Formerly IPV6_RTHDR, which is documented
.\"		IPV6_2292PKTOPTIONS (2.6.14)
.\"		    Formerly IPV6_PKTOPTIONS
.\"		IPV6_2292HOPLIMIT (2.6.14)
.\"		    Formerly IPV6_HOPLIMIT, which is documented
.\"
.\"		IPV6_RECVHOPLIMIT (2.6.14)
.\"		IPV6_RECVHOPOPTS (2.6.14)
.\"		IPV6_RTHDRDSTOPTS (2.6.14)
.\"		IPV6_RECVRTHDR (2.6.14)
.\"		IPV6_RECVDSTOPTS (2.6.14)
.\"
.\"	IPV6_RECVPATHMTU (Linux 2.6.35, flag value added in Linux 2.6.14)
.\"	    commit 793b14731686595a741d9f47726ad8b9a235385a
.\"	    Author: Brian Haley <brian.haley@hp.com>
.\"	IPV6_PATHMTU (Linux 2.6.35, flag value added in Linux 2.6.14)
.\"	    commit 793b14731686595a741d9f47726ad8b9a235385a
.\"	    Author: Brian Haley <brian.haley@hp.com>
.\"	IPV6_DONTFRAG (Linux 2.6.35, flag value added in Linux 2.6.14)
.\"	    commit 793b14731686595a741d9f47726ad8b9a235385a
.\"	    Author: Brian Haley <brian.haley@hp.com>
.\"	    commit 4b340ae20d0e2366792abe70f46629e576adaf5e
.\"	    Author: Brian Haley <brian.haley@hp.com>
.\"
.\"	IPV6_RECVTCLASS (Linux 2.6.14)
.\"	    commit 41a1f8ea4fbfcdc4232f023732584aae2220de31
.\"	    Author: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>
.\"		Based on patch from David L Stevens <dlstevens@us.ibm.com>
.\"
.\"	IPV6_CHECKSUM (Linux 2.2)
.\"	IPV6_NEXTHOP (Linux 2.2)
.\"	IPV6_JOIN_ANYCAST (Linux 2.4.21 / Linux 2.6)
.\"	IPV6_LEAVE_ANYCAST (Linux 2.4.21 / Linux 2.6)
.\"	IPV6_FLOWLABEL_MGR (Linux 2.2.7 / Linux 2.4)
.\"	IPV6_FLOWINFO_SEND (Linux 2.2.7 / Linux 2.4)
.\"	IPV6_IPSEC_POLICY (Linux 2.6)
.\"	IPV6_XFRM_POLICY (Linux 2.6)
.\"	IPV6_TCLASS (Linux 2.6)
.\"
.\"	IPV6_ADDR_PREFERENCES (Linux 2.6.26)
.\"	    commit 7cbca67c073263c179f605bdbbdc565ab29d801d
.\"	    Author: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>
.\"	IPV6_MINHOPCOUNT (Linux 2.6.35)
.\"	    commit e802af9cabb011f09b9c19a82faef3dd315f27eb
.\"	    Author: Stephen Hemminger <shemminger@vyatta.com>
.\"	IPV6_ORIGDSTADDR (Linux 2.6.37)
.\"	    Actually a CMSG rather than a sockopt?
.\"	    In header file, we have IPV6_RECVORIGDSTADDR == IPV6_ORIGDSTADDR
.\"	    commit 6c46862280c5f55eda7750391bc65cd7e08c7535
.\"	    Author: Balazs Scheidler <bazsi@balabit.hu>
.\"	IPV6_RECVORIGDSTADDR (Linux 2.6.37)
.\"	    commit 6c46862280c5f55eda7750391bc65cd7e08c7535
.\"	    Author: Balazs Scheidler <bazsi@balabit.hu>
.\"	        Support for IPV6_RECVORIGDSTADDR sockopt for UDP sockets
.\"		were contributed by Harry Mason.
.\"	IPV6_TRANSPARENT (Linux 2.6.37)
.\"	    commit 6c46862280c5f55eda7750391bc65cd7e08c7535
.\"	    Author: Balazs Scheidler <bazsi@balabit.hu>
.\"	IPV6_UNICAST_IF (Linux 3.4)
.\"	    commit c4062dfc425e94290ac427a98d6b4721dd2bc91f
.\"	    Author: Erich E. Hoover <ehoover@mines.edu>
.\"
.TH ipv6 7 2024-05-02 "Linux man-pages 6.9.1"
.SH NAME
ipv6 \- Linux IPv6 protocol implementation
.SH SYNOPSIS
.nf
.B #include <sys/socket.h>
.B #include <netinet/in.h>
.P
.IB tcp6_socket " = socket(AF_INET6, SOCK_STREAM, 0);"
.IB raw6_socket " = socket(AF_INET6, SOCK_RAW, " protocol ");"
.IB udp6_socket " = socket(AF_INET6, SOCK_DGRAM, " protocol ");"
.fi
.SH DESCRIPTION
Linux 2.2 optionally implements the Internet Protocol, version 6.
This man page contains a description of the IPv6 basic API as
implemented by the Linux kernel and glibc 2.1.
The interface
is based on the BSD sockets interface; see
.BR socket (7).
.P
The IPv6 API aims to be mostly compatible with the
IPv4 API (see
.BR ip (7)).
Only differences are described in this man page.
.P
To bind an
.B AF_INET6
socket to any process, the local address should be copied from the
.I in6addr_any
variable which has
.I in6_addr
type.
In static initializations,
.B IN6ADDR_ANY_INIT
may also be used, which expands to a constant expression.
Both of them are in network byte order.
.P
The IPv6 loopback address (::1) is available in the global
.I in6addr_loopback
variable.
For initializations,
.B IN6ADDR_LOOPBACK_INIT
should be used.
.P
IPv4 connections can be handled with the v6 API by using the
v4-mapped-on-v6 address type;
thus a program needs to support only this API type to
support both protocols.
This is handled transparently by the address
handling functions in the C library.
.P
IPv4 and IPv6 share the local port space.
When you get an IPv4 connection
or packet to an IPv6 socket,
its source address will be mapped to v6.
.SS Address format
.in +4n
.EX
struct sockaddr_in6 {
    sa_family_t     sin6_family;   /* AF_INET6 */
    in_port_t       sin6_port;     /* port number */
    uint32_t        sin6_flowinfo; /* IPv6 flow information */
    struct in6_addr sin6_addr;     /* IPv6 address */
    uint32_t        sin6_scope_id; /* Scope ID (new in Linux 2.4) */
};
\&
struct in6_addr {
    unsigned char   s6_addr[16];   /* IPv6 address */
};
.EE
.in
.P
.I sin6_family
is always set to
.BR AF_INET6 ;
.I sin6_port
is the protocol port (see
.I sin_port
in
.BR ip (7));
.I sin6_flowinfo
is the IPv6 flow identifier;
.I sin6_addr
is the 128-bit IPv6 address.
.I sin6_scope_id
is an ID depending on the scope of the address.
It is new in Linux 2.4.
Linux supports it only for link-local addresses, in that case
.I sin6_scope_id
contains the interface index (see
.BR netdevice (7))
.P
IPv6 supports several address types: unicast to address a single
host, multicast to address a group of hosts,
anycast to address the nearest member of a group of hosts
(not implemented in Linux), IPv4-on-IPv6 to
address an IPv4 host, and other reserved address types.
.P
The address notation for IPv6 is a group of 8 4-digit hexadecimal
numbers, separated with a \[aq]:\[aq].
\&"::" stands for a string of 0 bits.
Special addresses are ::1 for loopback and ::FFFF:<IPv4 address>
for IPv4-mapped-on-IPv6.
.P
The port space of IPv6 is shared with IPv4.
.SS Socket options
IPv6 supports some protocol-specific socket options that can be set with
.BR setsockopt (2)
and read with
.BR getsockopt (2).
The socket option level for IPv6 is
.BR IPPROTO_IPV6 .
A boolean integer flag is zero when it is false, otherwise true.
.TP
.B IPV6_ADDRFORM
Turn an
.B AF_INET6
socket into a socket of a different address family.
Only
.B AF_INET
is currently supported for that.
It is allowed only for IPv6 sockets
that are connected and bound to a v4-mapped-on-v6 address.
The argument is a pointer to an integer containing
.BR AF_INET .
This is useful to pass v4-mapped sockets as file descriptors to
programs that don't know how to deal with the IPv6 API.
.TP
.B IPV6_ADD_MEMBERSHIP, IPV6_DROP_MEMBERSHIP
Control membership in multicast groups.
Argument is a pointer to a
.IR "struct ipv6_mreq" .
.TP
.B IPV6_MTU
.BR getsockopt ():
Retrieve the current known path MTU of the current socket.
Valid only when the socket has been connected.
Returns an integer.
.IP
.BR setsockopt ():
Set the MTU to be used for the socket.
The MTU is limited by the device
MTU or the path MTU when path MTU discovery is enabled.
Argument is a pointer to integer.
.TP
.B IPV6_MTU_DISCOVER
Control path-MTU discovery on the socket.
See
.B IP_MTU_DISCOVER
in
.BR ip (7)
for details.
.TP
.B IPV6_MULTICAST_HOPS
Set the multicast hop limit for the socket.
Argument is a pointer to an
integer.
\-1 in the value means use the route default, otherwise it should be
between 0 and 255.
.TP
.B IPV6_MULTICAST_IF
Set the device for outgoing multicast packets on the socket.
This is allowed only for
.B SOCK_DGRAM
and
.B SOCK_RAW
socket.
The argument is a pointer to an interface index (see
.BR netdevice (7))
in an integer.
.TP
.B IPV6_MULTICAST_LOOP
Control whether the socket sees multicast packets that it has send itself.
Argument is a pointer to boolean.
.TP
.BR IPV6_RECVPKTINFO " (since Linux 2.6.14)"
Set delivery of the
.B IPV6_PKTINFO
control message on incoming datagrams.
Such control messages contain a
.IR "struct in6_pktinfo" ,
as per RFC 3542.
Allowed only for
.B SOCK_DGRAM
or
.B SOCK_RAW
sockets.
Argument is a pointer to a boolean value in an integer.
.TP
.B \%IPV6_RTHDR, \%IPV6_AUTHHDR, \%IPV6_DSTOPTS, \%IPV6_HOPOPTS, \
\%IPV6_FLOWINFO, \%IPV6_HOPLIMIT
Set delivery of control messages for incoming datagrams containing
extension headers from the received packet.
.B IPV6_RTHDR
delivers the routing header,
.B IPV6_AUTHHDR
delivers the authentication header,
.B IPV6_DSTOPTS
delivers the destination options,
.B IPV6_HOPOPTS
delivers the hop options,
.B IPV6_FLOWINFO
delivers an integer containing the flow ID,
.B IPV6_HOPLIMIT
delivers an integer containing the hop count of the packet.
The control messages have the same type as the socket option.
All these header options can also be set for outgoing packets
by putting the appropriate control message into the control buffer of
.BR sendmsg (2).
Allowed only for
.B SOCK_DGRAM
or
.B SOCK_RAW
sockets.
Argument is a pointer to a boolean value.
.TP
.B IPV6_RECVERR
Control receiving of asynchronous error options.
See
.B IP_RECVERR
in
.BR ip (7)
for details.
Argument is a pointer to boolean.
.TP
.B IPV6_ROUTER_ALERT
Pass forwarded packets containing a router alert hop-by-hop option to
this socket.
Allowed only for
.B SOCK_RAW
sockets.
The tapped packets are not forwarded by the kernel, it is the
user's responsibility to send them out again.
Argument is a pointer to an integer.
A positive integer indicates a router alert option value to intercept.
Packets carrying a router alert option with a value field containing
this integer will be delivered to the socket.
A negative integer disables delivery of packets with router alert options
to this socket.
.TP
.B IPV6_UNICAST_HOPS
Set the unicast hop limit for the socket.
Argument is a pointer to an integer.
\-1 in the value means use the route default,
otherwise it should be between 0 and 255.
.TP
.BR IPV6_V6ONLY " (since Linux 2.4.21 and 2.6)"
.\" See RFC 3493
If this flag is set to true (nonzero), then the socket is restricted
to sending and receiving IPv6 packets only.
In this case, an IPv4 and an IPv6 application can bind
to a single port at the same time.
.IP
If this flag is set to false (zero),
then the socket can be used to send and receive packets
to and from an IPv6 address or an IPv4-mapped IPv6 address.
.IP
The argument is a pointer to a boolean value in an integer.
.IP
The default value for this flag is defined by the contents of the file
.IR /proc/sys/net/ipv6/bindv6only .
The default value for that file is 0 (false).
.\" FLOWLABEL_MGR, FLOWINFO_SEND
.SH ERRORS
.TP
.B ENODEV
The user tried to
.BR bind (2)
to a link-local IPv6 address, but the
.I sin6_scope_id
in the supplied
.I sockaddr_in6
structure is not a valid
interface index.
.SH VERSIONS
Linux 2.4 will break binary compatibility for the
.I sockaddr_in6
for 64-bit
hosts by changing the alignment of
.I in6_addr
and adding an additional
.I sin6_scope_id
field.
The kernel interfaces stay compatible, but a program including
.I sockaddr_in6
or
.I in6_addr
into other structures may not be.
This is not
a problem for 32-bit hosts like i386.
.P
The
.I sin6_flowinfo
field is new in Linux 2.4.
It is transparently passed/read by the kernel
when the passed address length contains it.
Some programs that pass a longer address buffer and then
check the outgoing address length may break.
.SH NOTES
The
.I sockaddr_in6
structure is bigger than the generic
.IR sockaddr .
Programs that assume that all address types can be stored safely in a
.I struct sockaddr
need to be changed to use
.I struct sockaddr_storage
for that instead.
.P
.BR SOL_IP ,
.BR SOL_IPV6 ,
.BR SOL_ICMPV6 ,
and other
.B SOL_*
socket options are nonportable variants of
.BR IPPROTO_* .
See also
.BR ip (7).
.SH BUGS
The IPv6 extended API as in RFC\ 2292 is currently only partly
implemented;
although the 2.2 kernel has near complete support for receiving options,
the macros for generating IPv6 options are missing in glibc 2.1.
.P
IPSec support for EH and AH headers is missing.
.P
Flow label management is not complete and not documented here.
.P
This man page is not complete.
.SH SEE ALSO
.BR cmsg (3),
.BR ip (7)
.P
RFC\ 2553: IPv6 BASIC API;
Linux tries to be compliant to this.
RFC\ 2460: IPv6 specification.
